The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly, to a semiconductor device having a wiring layer structure and a method for manufacturing the same.
In less-dense conventional semiconductor devices, metal step coverage has not been a serious problem. However, recently, with increased integration density in semiconductor devices, the diameter of contact holes has become significantly smaller, to half-micron dimensions, while the aspect ratio thereof has become much larger and the impurity-doped regions formed in the semiconductor substrate have become much shallower. Accordingly, it has become necessary to improve on the conventional method for forming a wiring layer structure using aluminum (Al), since filling up a contact hole of 0.5.mu.m or below having an aspect ratio of two or greater is difficult and metal wiring layer structure reliability will be deteriorated due to void formation. These days, the wiring method for a semiconductor device is regarded as being most important in the process semiconductor manufacturing process, since it determines the speed, yield, and the reliability of the semiconductor device.
To solve such problems as void formation caused by the high aspect ratio and the poor step coverage of the sputtered aluminum, a method for filling up the contact hole by melting aluminum is proposed. For example, Japanese Laid-open Publication No. 62-132348 (by Yukiyasu Sugano et al.), Japanese Laid-open Publication No. 63-99546 (by Shinpei Iijima et al.), and Japanese Laid-open Publication No. 62,109341 (by Misahiro Shimizu et al.) disclose a melting method. According to the above publications, the contact hole is filled up by depositing aluminum or an aluminum alloy and heating aluminum beyond its melting point and then, reflowing the liquid aluminum. In addition to the above disclosures, U.S. Pat. Nos. 4,920,027 (by Ryoichi Mukai), 4,800,179 (by Ryoichi Mukai) and 4,758,533 (by Magee Thomas et al.) disclose a method for forming a planarized metal layer by depositing the metal layer and heating to melt it using a laser beam.
According to the above methods, the semiconductor wafer has to be disposed horizontally so as to allow proper filling of the contact hole with the flowing melted aluminum. Then, the liquid metal layer will try to seek a lower surface tension, and thus, may shrink or warp when solidified, thereby exposing the underlying semiconductor material. Further, the heat treatment temperature cannot be precisely controlled and therefore, it is difficult to reproduce the desired results. Moreover, the areas of the metal layer beside the contact hole become rough, which causes problems in the subsequent photolithography process.
A multiple step metallization process is disclosed in U.S. Pat. No. 4,970,176 (by Tracy et al.), wherein a thick metal layer having a predetermined thickness is deposited on a semiconductor wafer at a low temperature (below 200.degree. C. approximately). Then, the remaining and relatively thin metal layer is deposited on a semiconductor wafer while the temperature is increased to approximately 400.degree. C to 500.degree. C. The reflow of the deposited metal layer takes place through grain growth, re-crystallization and bulk diffusion, which improves the step coverage of the metal layer.
However, a contact hole whose diameter is 1.mu.m or below cannot be completely filled up by aluminum or an aluminum allow. Therefore, the above method is not suitable for filling up the contact hole of a highly integrated semiconductor device.
Meanwhile, Ono et al. have disclosed that when the temperature of Al-Si film is above 500.degree. C., the liquidity of Al-Si suddenly increases. They disclosed a method that fills up the contact hole by depositing Al-Si film at a temperature of 500-550.degree. C. (see Hisako Ono et al., in Proc. 1990 VMIC Conference June 11-12, pp. 76-82). U.S. Pat. No. 5,071,791 (by Inoue Minoru et al.) discloses a method for forming a wiring layer which has a good step coverage and a planarized surface by depositing an Al alloy and then heating the wafer beyond a predetermined temperature.
Additionally, Yoda Dakashi et al. have suggested a method for filling up the contact hole by depositing metal at a temperature of 500-550 (European Pat. application No. 80104184.8 corresponding to Japanese Pat. application No. 89-61557). According to the Yoda Dakashi method, the contact hole may be completed filled up by the metal. However, there is a high probability that the Al-Si film has a strong resistance against the electron migration but a weak resistance against stress. In addition to this, Si included in Al film is crystallized at the interfaces between Al-Si grains, which is undesirable. Thus, it is necessary to remove the Al-Si film at the areas other than the contact hole area, and the metallization process becomes complicated.
Additionally, C.S. Park et al. (which includes the present inventor) have disclosed a method for forming a metal wiring layer which comprises the steps of depositing an aluminum alloy at a low temperature of 100.degree. C. or below, and performing a heat treatment for approximately three minutes at a temperature of 550.degree. C., i.e., a temperature below the melting point, to thereby completely fill up the contact hole with aluminum alloy (see Proc. 1991 VMIC Conference, June 11 and 12, pp. 326-328). This method is included in U.S. Patent application Ser. No. 07/897,294 now U.S. Pat. No. 5,318,923, filed on Aug. 24, 1992 (as a continuation-in-part of U.S. Pat. application Ser. No. 07/585,218 entitled "A Method for Forming a Metal Layer in a Semiconductor Device" and filed on Sept. 19, 1990). The aluminum deposited at a low temperature is not melted during heat treatment at 550.degree. C., but migrates into the contact hole, thereby completely filling the contact hole.
According to the C.S. Park et al. method, the contact hole, having a size of 0.8.mu.m and an aspect ratio of approximately 1.0, can be completely filled up by performing a heat treatment even after aluminum is deposited to a thickness of approximately 500.ANG. at a low temperature of 100.degree. C. or below. This method does not require the etching process described in the Yoda Dakashi et al. patent. In addition, this method can fill up the contact hole by forming a very thin Al film and then heat-treating. Therefore, the smaller-sized contact holes expected in the future can be filled up with Al or Al alloy in the recent semiconductor process where the size of the contact hole is becoming smaller. For these advantages, a method for filling up the contact hole by the C.S. Park method is attracting a great deal of interest in the pertinent art.
As described above, Al or an Al alloy is generally deposited by a sputtering method which can produce a wiring layer having a good resistance against electro-migration. However, the sputtering method has a step coverage problem, as described above, and the development of other depositing methods thus become necessary. Methods where a void is completely filled by depositing Al using a chemical vapor deposition (CVD) method, have been suggested. For example, U.S. Pat. Nos. 4,460,610 and 4,433,012 (both by Heinecke et al.) disclose a method for depositing Al by thermally decomposing a triisobutyl aluminum (hereinafter referred to as "TIBA") through a CVD method.
The representative method for depositing Al using the CVD method is that, as described above, an organic Al precursor is volatilized and then thermally decomposed to thereby deposit Al. Representative organic Al precursors include TIBA, dimethyl aluminum hydride (DMAH, (CH.sub.6).sub.2 AlH) etc.
In general, when Al is deposited by a low-pressure CVD method using TIBA, at the substrate temperature of 260.degree. C., while maintaining the vapor temperature of TIBA at 45.degree. C. and using argon as a carrier gas, and at the pressure of approximately 1Torr, the Al deposition rate is 1,500.ANG./minute.
Referring to the method of depositing Al using the CVD method, the step coverage of CVD-deposited aluminum (hereinafter referred to as "CVD-Ai") is excellent. However, the surface of the obtained CVD-Al layer is rough which causes problems in the subsequent lithography process. Further, the wiring layer formed from the CVD-Al layer has a very poor resistance against electron-migration and the reliability thereof is insufficient. Moreover, Al cannot be deposited in the form of Al-Si alloy by a CVD method. (see Silicon Processing for the VLSi Era, Vol. 2, by S. Wolf, p. 254).
In order to improve the uniformity of the obtained CVD-Al film, a method for promoting the nucleation of Al in the CVD method by exposing the wafer to a TiCl.sub.4 atmosphere to thereby pretreat the surface of the substrate was proposed (see U.S. Pat. No. 4,460,618). Also, a method which uses Al hydrides such as DMAH, AlH.sub.2 (i-C.sub.4 H.sub.4) or AlH.sub.2 Cl as a source was proposed (see U.S. Pat. No. 3,462,288).
Additionally, U.S. Pat. No. 4,328,261 discloses a method for depositing Al, using silane and Al alkyl gas as a source, by a CVD method at a high temperature and at a low pressure, so as to form a film comprised of Al-Si alloy.
U.S. Pat. No. 4,923,717 discloses a method for forming Al film. A mirror-like Al is deposited on the surface of the substrate, the substrate is treated using an IVB or VB group metal complex compound such as TICl.sub.4, and then an Al hydride is decomposed to thereby form the Al film. However, by using the method of pretreatment including TICl.sub.4, the remaining chlorine causes a corrosion problem.
As stated above, since the process for depositing Al using a CVD method has many problems, it is not widely used.
It is known that a diffusion barrier layer can be formed between the wiring layer and the silicon wafer or an insulating layer, so as to prevent Al spiking. Si precipitates and Si-nodule formation is caused by the above-mentioned reaction between the metal wiring layer and the silicon wafer. For example, U.S. Pat. No. 4,897,709 (by Yokoyama et al.) describes a method for forming a nitride titanium film as a diffusion barrier layer on the inner walls of the contact hole. Additionally, in Japanese Laid-open Publication No. 61-183942, a method is described for forming a barrier layer which comprises the steps of forming a refractory metal layer, depositing a metal such as Mo, W, Ti or Ta, forming a titanium nitride layer on the refractory metal layer and heat-treating the double layer which consists of a refractory metal layer and a nitride titanium layer to thereby form a refractory metal silicide layer consisting of thermally stable compounds at the inter-surface of the refractory metal layer and semiconductor substrate by a reaction therebetween. Thus, the barrier characteristic is improved. This heat treatment of the diffusion barrier layer is performed by an annealing process under a nitrogen atmosphere. When the diffusion barrier layer does not undergo the annealing process, junction spiking occurs in a subsequent sintering step after Al sputtering, or during sputtering Al or an Al alloy at a temperature above 450.degree. C., which is undesirable.
Additionally, Hagita Masafumi has suggested a method wherein a TiN layer as a barrier layer is heat-treated, and then, the barrier layer is implanted with O.sub.2 or silicon in order to improve the wettability between the barrier metal and the Al wiring and to improve the quality and yield of the wiring (Japanese Laid-open Publication No. 2-26052).
Also, Nishima Kenji et al. have suggested a method for improving a barrier characteristic upon forming a diffusion barrier layer. This method comprises the steps of forming a TiN layer and then heat-treating, and forming a TiN layer again (Japanese Laid-open Publication No. 63-97762).
Besides the method for preventing Al spiking or Si precipitate crystallization by improving the characteristic of a diffusion barrier layer as described above, a method for preventing Al spiking or Si precipitates by forming a composite layer having various compositions as an Al wiring layer has also been suggested.
For example, a method for preventing Si-precipitates in a sintering process when a wiring layer is formed is disclosed in Japanese Laid-open Publication No. 2-159065 (by Michiichi Masmoto). This method comprises the steps of forming an Al-Si film first and then forming a pure Al layer, thereby preventing Si-precipitates in the sintering process. Further, U.S. Pat. application Ser. Nos. 07/828,458 (filed on Jan. 31, 1992 now U.S. Pat. No. 5,266,521 and 07/910,984 (filed on July 8, 1992 now U.S. Pat. No. 5,355,020 by S.I. Lee (the present inventor) et al. disclose a method for forming a composite layer so as to prevent the crystallization of Si precipitates generated when the contact hole is filled by depositing Al at a low temperature and heat-treating at a high temperature below the melting point according to the C.S. Park et al. method.
Generally, in order to form a metal layer after forming a diffusion barrier layer, the wafer is exposed to the atmosphere since the wafer should be transferred to sputtering equipment to form the metal layer.
At this time, oxidation occurs in the interfaces of the grains or in the surface portion of the diffusion barrier layer, and the mobility of aluminum atoms on the oxidized diffusion barrier layer is decreased. When an Al-1% Si-0.5% Cu alloy is deposited to a thickness of 6,000.ANG. at room temperature, the formed grains are small, i.e., approximately 0.2.mu.m.
Meanwhile, large grains of up to approximately 1m are formed on the diffusion barrier layer unexposed to the atmosphere. Aluminum reacts with the diffusion barrier layer during a heat-treating step at a high temperature or when depositing an Al film by sputtering at a high temperature, to thereby make the surface of the Al film very rough, which deteriorates surface reflectivity. As a result, the subsequent photolithography process is difficult to perform.
In general, a titanium nitride (TIN) layer or TiW (or TiW(N)) layer is used as the diffusion barrier layer. Such layers have micro-structured defects or grain boundaries which cannot prevent silicon or Al diffusion at the grain boundary when forming a thin film of the diffusion barrier layer. A method for blocking a diffusion path in the grain boundary according to the "oxygen stuffing" method has been suggested. When the diffusion barrier layer is exposed to a N.sub.2 annealing process or to the atmosphere, a small amount of oxygen is mixed into the barrier layer, to thereby increase the diffusion barrier effect. This is called the "stuffing effect."
Generally, when TiN is deposited and exposed to the atmosphere, a stuffing effect occurs due to the oxygen in the atmosphere. The method of the Hagita patent also oxygenates the surface of the diffusion barrier layer, thereby improving the characteristics of the barrier metal.
However, the contact resistance can be increased when Ti or TiN is deposited so as to form a barrier layer which is then exposed to the atmosphere, when TiN is deposited while introducing the oxygen, or when the barrier layer is annealed under the nitrogen atmosphere wherein oxygen is introduced.
It is desirable to form an oxide on the surface of the diffusion barrier layer and in the grain boundary thereof, to improve the characteristics of the diffusion barrier layer in the contact hole. However, this oxide may deteriorate the wettability of the diffusion barrier layer and the Al so that a void may be formed in the contact hole, or a metal layer which has a poor profile during heat treatment may be formed, which thereby deteriorates the reliability of the wiring layer of the semiconductor device.
FIGS. 1 to 3 illustrate defects of a wiring layer which can be generated when the contact hole is filled up by depositing Al using the conventional method.
FIGS. 1 to 3, reference numeral 1 is a semiconductor wafer, reference numeral 2 is an impurity-doped region, reference numeral 3 is an insulating layer (BPSG film), reference numeral 4 is a diffusion barrier layer, and reference numeral 6 is an Al alloy metal layer. FIG. 1 illustrates the profile of the deposited Al obtained by a conventional sputtering method. FIG, 2 illustrates an Al layer 6 obtained by depositing Al using the above CVD method, and FIG. 3 illustrates a void 7 in the contact hole when Al is deposited and heat-treated in a vacuum or when the contact hole is filled up by a high-temperature sputtering method.
As described above, according to the conventional method of FIG. 1, it is very hard to fill up the contact hole which has a high aspect ratio and whose size is below one-half micron, since the step coverage of the sputtered Al is poor. The reliability of the CVD-Al layer is unacceptable, which causes difficulty in applying it to an actual semiconductor device. According to a method wherein the contact hole is filled up by depositing Al at a low temperature and heat-treated in a vacuum, the deposition and heat-treatment steps have to be performed repeatedly so as to fill up the small and deep contact hole. Therefore, the throughput is lowered.